Ink jet recorders or printers have been the subject of an intense development effort for a number of years. The recorders fall generally into two categories, namely drop-on-demand and continuous stream types. The drop-on-demand ink jet printers in general emit an individual drop of ink as needed to form a print image. The present invention is applicable to the continuous stream type of recorder.
In general, a continuous stream ink jet printer pumps ink under high pressure through a restricted orifice or nozzle. The stream issuing from the nozzle separates into fine ink droplets, which are directed toward a recording medium, such as a paper sheet. The control of the ink stream to "paint" the recording surface is accomplished in several ways. In one variety of continuous stream printers, the ink jet passes into a deflection unit wherein portions of the ink jet, which are not intended to impinge the recording surface, are selectively deflected into a sump or gutter or towards non-critical areas of the medium, such as a margin, or even a member used to support the medium. The undiverted portions of the stream impact the recording medium and thereby "paint" the desired image on the medium. The deflection of the selected portions of the ink jet results in the creation of an ink mist. However, this mist and the resulting condensate can be substantially eliminated by means of pumps as disclosed in co-pending application Ser. No. 753,454. Another variety employs apparatus, such as a valve within the nozzle unit, which interrupts the stream flow to cause a break in the ink jet, while selectively permitting the ink stream to issue from the nozzle as needed.
Due to the high pressure used to expel the ink stream in continuous ink jet printers, the undiverted or uninterrupted droplets striking the recording medium impact with a considerable force. This impact causes the droplets to disintergrate. While most of the ink adheres to the medium, a significant amount rebounds from off the medium in the form of minute particles. This results in an ink mist adjacent to the recording surface.
This ink mist can, if not adequately dealt with, cause significant problems in the overall effectiveness and utility of an ink jet printer. Most importantly the mist causes problems with cleanliness and efficiency of operation. For example, a substantial portion of the mist condenses on the nozzle assembly. This results in increased maintenance expense. The ink mist may also become entrained in the ink stream droplets approaching the recording medium, thereby causing background coloration, particularly, in ink jet printers using multiple ink colors. There is also evidence that the ink mist may contaminate the atmosphere surrounding the printer, including areas external to a housing which is often used to contain the printer.
Prior attempts to solve the latter ink mist problem have included pump arrangements which are essentially designed to suck the ink mist directly from the atmosphere adjacent to the recording surface. These arrangements have not effectively eliminated the problem. Considerable ink still condenses throughout the printer, causing the aforementioned cleanliness and efficiency problems, as well as adversely effecting the quality of the printed image. Further, they have not eliminated the contamination of the work area around the ink jet printer. Moreover, the blowers necessary to accomplish this suction are expensive, require frequent filter replacement or cleaning and require excessive space in the printer.
Another solution disclosed in the co-pending application Ser. No. 753,454 comprises the charging of a mist shield positioned adjacent the recording surface to attract the uncharged, electrical conductive ink particles which rebounds from the recording surface. This also has not proven entirely satisfactory in that over time dust or other foreign matter builds upon the mist shield and the dust traps the rebounding ink mist in a manner where this ink mist interferes with the ink stream and also condenses on the dust particles and falls away into undesirable locations.